This invention relates generally to ohmic contact metallization on n-type Group III-V semiconductors and, more particularly, to contacts formed on semiconductors such as gallium arsenide (GaAs).
The reliability of GaAs devices, such as field effect transistors, mixer and IMPATT diodes, and Gunn devices, is directly related to the integrity of the low-resistance ohmic contacts to the semiconductor surface. Typically, present contacting methods involve depositing a solid solution of gold and germanium or silver and germanium on a GaAs substrate and then applying sufficient heat to melt the metals by annealing through the eutectic temperature and thereby alloy the metal film with the substrate. Such methods which involve melting produce ohmic contacts which are adequate for many purposes. However, major disadvantages of the melting processes are the lack of uniform wetting of the metals to GaAs, microsegregation and microcrystallization of the metals and GaAs, nonuniform germanium doping characteristics in the semiconductor regrown layer, outdiffusion of gallium and arsenic from the GaAs to the top layer of the contact, and germanium surface roughness. These problems adversely affect the reliability, electrical performance, and dimensional control of the contact metallization.